Fine particles having diameter between 1 nm and 10 μm may be present in inside air of residential building and rooms, office premises and also in industrial buildings as well as in outdoor air and open-air. These fine particles may be formed for example in different kind of combustion processes, industrial processes and also in natural sources. For various reasons these fine particles are measured. The fine particle measurements may be conducted because of their potential health effects and also for monitoring operation of combustion or industrial processes, such as operation of combustion engines, especially diesel engines. The fine particles are also measured in ventilation systems for monitoring air quality. Another reason for monitoring fine particles is the increasing use and production of nanosized particles in industrial processes. The above reasons generate a need for reliable fine particle measurement apparatuses and methods.
Prior art methods and apparatuses for measuring particles comprise measuring fine particles electrically utilizing electrical charging of the fine particles.
In a prior art apparatus clean, essentially particle free, gas is supplied into the apparatus. The clean gas is further ionized before and during supplying it into the apparatus. The prior art apparatus may comprise a corona charger connected to corona voltage source for ionizing the clean gas. The apparatus further comprises sample inlet via which particle containing aerosol sample is supplied into the apparatus. Inside the apparatus the ionized clean gas flow and the particle containing sample aerosol flow are brought into contact with each other and mixed such that a mixed sample aerosol flow is generated. The ionized clean gas electrically charges the particles of the sample aerosol flow by diffusion charging upon contact and mixing of the ionized clean gas and the particle containing sample aerosol flow.
Free ions, meaning ions not attached to particle, may be removed from the mixed sample aerosol flow using ion trap. The ion trap may be an ion trap electrode connected to an ion trap voltage source providing collection voltage for removing the free ions from the mixed sample aerosol flow. The ion trap voltage is high enough to deposit the free ions, but due to the lower mobility charged particles are not deposited.
The mixed sample aerosol flow comprising the electrically charged particles them flows to a filter unit in which the electrically charged particles are collected by a filter. The particle measurement is carried out by measuring the electrical current in the filter generated by the collected electrically charged particles for example with an electrometer. Downstream of the filter unit is arranged a pump or other suction device for providing the clean gas flow and the sample aerosol flow through apparatus.
In many applications the apparatus for particle measurement has to be located in small spaces. Therefore size of the apparatus should be as small as possible for efficient utilization of space. However, decreasing the dimensions of the apparatus may reduce quality of the measurements as the flow dynamics of the sample aerosol are compromised by the decreased dimension. Therefore, for the prior art apparatuses for particles measurement the balance between the dimensions and the measurement quality are a compromise. Small dimensions deteriorate measurement quality and large dimensions cause inefficient space utilization.